Dummy fill to reduce shallow trench isolation (STI) stress variation on transistor performance

ABSTRACT

A method of forming an integrated circuit structure on a chip includes extracting an active layer from a design of the integrated circuit structure, forming a guard band conforming to the shape of the active layer, the guard band surrounds the active layer, and the guard band is spaced from the active layer at a first spacing in the X-axis direction and at a second spacing in the Y-axis direction, removing any part of the guard band that violates design rules, removing convex corners of the guard band, and adding dummy diffusion patterns into the remaining space of the chip outside the guard band. The first and second spacing can be specified as the same spacings in a Spice model characterization of the integrated circuit structure. The dummy diffusion patterns with different granularities can be added so that the diffusion density is substantially uniform over the chip.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is based on, and claims priority from, U.S.Provisional No. 61/156,344 filed Feb. 27, 2009, entitled “Dummy Fill toReduce Shallow Trench Isolation (STI) Stress Variation on TransistorPerformance”, the disclosure of which is hereby incorporated byreference herein in its entirety.

RELATED APPLICATION

This application incorporates by reference U.S. patent application Ser.No. 12/211,503, filed Sep. 16, 2008, entitled “Dummy Pattern Design forReducing Device Performance Drift.”

TECHNICAL FIELD

The disclosure relates generally to integrated circuits, and moreparticularly to metal-oxide-semiconductor (MOS) devices, and even moreparticularly to a dummy pattern design for reducing the performancedrift of the MOS devices caused by differences in stresses applied onthe MOS devices.

BACKGROUND

It is well known that the drive currents of metal-oxide-semiconductor(MOS) devices are affected by the stresses applied on their channelregions. The stresses in the channel regions may improve or degrade thecarrier mobility depending on the direction of stresses. Generally, itis desirable to induce a tensile stress in the channel region of ann-type MOS (NMOS) device, and to induce a compressive stress in thechannel region of a p-type MOS (PMOS) device, e.g. for <110> channel on(100) Si substrate.

Although the beneficial stresses in the channel regions are generallydesirable, it is also realized that the magnitude of the drive currentimprovement is related to the magnitude of the stress. Further, anincorrect stress direction, such as compressive stress in the transversedirection, may degrade the mobility and subsequently the current, whichshould be minimized. On a same semiconductor chip, the MOS devices maybe applied with stresses having different magnitudes. Accordingly, thedrive current improvements or degradations for different MOS devices maybe different, resulting in non-uniform drive currents, and hencenon-uniform drive current drifts.

For each of the MOS devices in a semiconductor chip, the respectivespacing from other MOS devices affects its performance. The spacing maybe filled with shallow trench isolation (STI) regions (or fieldregions). Due to the inherent stress of the insulation materials, theSTI regions apply stresses to adjacent MOS devices, and the magnitudesof the stresses are affected by the spacing. The variations in thespacing cause variations in the stresses generated by STI regions.Therefore, it is difficult to predict and compensate for the drivecurrent drifts in circuit simulations.

The performance of MOS devices needs to be predictable, so that atcircuit design time, simulations may accurately reflect the circuitbehavior. Accordingly, it is preferred that in a semiconductor chip atleast the MOS devices of a same type circuit have a uniform performance.In the situation of non-uniform drive current drift, the drive currentdrift has to be compensated for during the simulations of the circuitdesign. What makes the compensation of the drive current driftcomplicated is that the stresses of MOS devices are affected by variousfactors and those factors behave differently for different layouts.

Conventional integrated circuit designs, however, often neglected suchissues. For example, U.S. Pat. No. 5,278,105 provides a method foradding dummy regions. The method includes extracting layouts of activelayers, forming blocked regions including the patterns of the activelayers, and laying out dummy patterns in regions other than the blockedregions. The primary purpose of this method is to improve diffusiondensity for chemical-mechanical polishing (CMP) or loading effect ofetching. This method places dummy diffusion patterns randomly outsidethe “block” layer or “keep-out region.” However, this method may havevarious shallow trench isolation (STI) widths even for simplerectangular diffusions as explained in the following.

FIG. 1 illustrates a conventional layout of an integrated circuit havingactive and dummy diffusion regions. In FIG. 1, an example layout isshown including active regions 2, 4 and 6 (active diffusion area), gateelectrode strips 8, 10 and 12 (polysilicon area), and dummy regions 14(dummy diffusion area). One skilled in the art will appreciate that thegate strips 8, 10, and 12 may be formed of materials other thanpolysilicon, such as metals, metal silicides, metal nitrides,polysilicon, and combinations thereof. Active region 2 and the overlyinggate electrode strip 8 belong to MOS device 18, while active region 4and the overlying gate electrode strip 10 belong to MOS device 20. It isnoted that one of the dummy layer in regions 14 is spaced apart fromactive region 2 by spacing S1. Accordingly, the paths for applyingstress (referred to as stress-application paths hereinafter) by STIregions 16 have a length S1. Similarly, one of the dummy layers inregions 14 is spaced apart from active region 4 by spacing S1. However,along another stress-application path, the stress-application path mayhave length S2 or S3, both different from S1. The significant differencein the lengths of the stress-application paths results in a largevariation in the stresses applied by STI in region 16, and hence in asignificant variation in the performance (for example, drive currents)of MOS devices 18 and 20. STI stress effect affects transistorperformance parameters (such as I_(d-lin), V_(t), I_(leak), I_(d sat)),and I_(d sat) can be about 15˜20% off from a Spice model of theintegrated circuits. For example, with a greater stress-applicationlength S2, STI in region 16 may apply a greater stress to the channelregion of MOS device 20 than the stress applied to the channel region ofMOS device 18. The device drive current drift between MOS devices 18 and20 may reach about 10 to 20 percent.

The U.S. patent application Ser. No. 12/211,503, filed Sep. 16, 2008 andentitled “Dummy Pattern Design for Reducing Device Performance Drift”describes added dummy diffusion regions for blocking stress-applicationpaths to reduce the variations in the stresses applied to MOS devices.One described method is to add dummy diffusion stripes parallel abuttingto the “block layer” and add general dummy diffusion patterns to meet agiven density target. However, even though this method can make aconstant STI width for simple rectangular diffusion patterns, it failsto control STI widths for non-rectangular diffusion patterns, asexplained in the following.

FIG. 2 illustrates another layout of an integrated circuit having dummydiffusion stripes and general dummy diffusion patterns. In FIG. 2, anexample layout is shown including block layers 202, 204, 206, and 208,where each block layer surrounds active regions 210, 212, 214, and 216inside, and in turn dummy diffusion stripes 218 and general dummydiffusion patterns 14 surround each block layer. For the block layer 202and 204, the spacings between the active regions (210 and 212) and dummydiffusion stripes 218 are constant, i.e. S_(L1) in the X direction andS_(w1) in the Y direction. Also for the block layer 206, the spacingsbetween the active region 214 and dummy diffusion stripes 218 areconstant for each X- and Y-direction i.e. S_(L2) and S_(w2)respectively, due to the rectangular shape of the active region 214inside. However, for the block layer 208, there are substantialvariations in the spacings between the active region 216 and dummydiffusion stripes 218 and/or general dummy diffusion patterns 14 in bothX- and Y-directions, i.e. S_(L3), S_(L4) and S_(L5) in the X-direction,and S_(w3), S_(w4) and S_(w5) in the Y-direction. This is due to theirregular (non-rectangular) shape of active region 216 inside therectangular shape of the blocked region 208. The significant differencein the lengths of the stress-application paths (spacings) results in alarge variation in the stresses applied by STI in region 16, and hencein a significant variation in the performance (for example, drivecurrents) of MOS devices inside the block layer 208.

Accordingly, new methods for well controlled STI widths or oxidedefinition (OD) spacing and significantly reduced device performancevariations (e.g. drive current drifts) of MOS devices regardless ofdiffusion shapes are needed.

SUMMARY OF THE INVENTION

In accordance with one or more embodiments, a method of forming anintegrated circuit structure on a chip includes extracting an activelayer from a design of the integrated circuit structure, where theactive layer comprises an active pattern having a diffusion region;forming at least one guard band conforming to the shape of the activelayer, where the guard band is a dummy diffusion layer, the guard bandsurrounds the active layer without a break, and the guard band is spacedfrom the active layer at a first spacing in the X-axis direction and ata second spacing in the Y-axis direction; removing any part of the guardband that violates design rules; removing convex corners of the guardband; and adding dummy diffusion patterns into the remaining space ofthe chip outside the guard band. The guard band in the method can have auniform width or specified widths. Also, dummy diffusion patterns in themethod can have different granularities in sizes. The dummy diffusionpatterns can be added so that the diffusion density is substantiallyuniform over the chip.

In one embodiment, the method may include adding a block layer beforeforming the guard band, where no dummy diffusion layers are added insidethe block layer. In another embodiment, the first spacing can bespecified as the same spacing in the X-direction used in a Spice modelcharacterization of the integrated circuit structure and/or the secondspacing can be specified as the same as the spacing in the Y-directionused in a Spice model characterization of the integrated circuitstructure. In yet another embodiment, the method may include cutting theguard band longer than a specified length, where the length isdetermined by design rules.

In accordance with one or more embodiments, a method of forming anintegrated circuit structure on a chip includes extracting an activelayer from a design of the integrated circuit structure, where theactive layer comprises an active pattern having a diffusion layer;forming at least one guard band conforming to a shape of the activelayer, where the guard band is a dummy diffusion layer, the guard bandsurrounds the active layer, and the guard band is spaced from the activelayer at a first spacing in the X-axis direction and at a second spacingin the Y-axis direction, and there is no guard band at locations whereconvex corners are necessary in order to have a continuous guard band;and adding dummy diffusion patterns into the remaining space of the chipoutside the guard band.

The advantageous features of the present invention include wellcontrolled STI widths or oxide definition (OD) spacing of devices,actual diffusion patterns having the same X- and Y-diffusion spacings asa Spice model characterization spacings if the same spacings arespecified, and significantly reduced device performance variations dueto reduced STI stress effect variations regardless of diffusion shapes,as well as reducing the gap between pre-layout and post-layoutsimulation and maximizing Silicon versus simulation correlations.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a conventional layout of an integrated circuit havingactive and dummy diffusion layers;

FIG. 2 illustrates another layout of an integrated circuit having dummydiffusion stripes and general dummy diffusion patterns;

FIG. 3 illustrates an example layout according to one aspect of thedisclosed embodiments, having guard bands conforming to the shapes ofthe active diffusion layers; and

FIG. 4 through 6 are top views of intermediate stages according to oneaspect of a method of adding dummy diffusion patterns, where the guardbands are conforming to the shapes of the active layers, and blocklayers are used to keep out dummy diffusion patterns at specifiedspacings from active layers.

DETAILED DESCRIPTION OF EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

A method of inserting dummy patterns into layouts of integrated circuitsis provided. The intermediate stages of the method are provided, and thevariations of the method are also discussed. Throughout the variousviews and illustrative embodiments of the present invention, likereference numbers are used to designate like elements.

FIG. 3 illustrates an example layout according to one aspect of thedisclosed embodiment, having guard bands conforming to the shapes of theactive layers. The guard bands are dummy diffusion layers surroundingactive layers. In FIG. 3, active layers including active diffusionregions 210, 212, 214, and 216 (extracted from a design of theintegrated circuit structure), are surrounded by guard bands 302 thatare conforming to the shapes of the active layers. The guard bands 302surround the active regions without a break and are spaced from theactive regions at a spacing S_(L) in the X-axis direction and at aspacing S_(W) in the Y-axis direction.

The guard bands 302 have a uniform width in correlation with a Spicemodel or various specified widths for device matching. The region 303 isan example of a portion of the guard band 302 that is removable if itviolates design rules (DR). As shown in FIG. 3, the spacing between theactive regions (210, 212, 214, and 216) and the guard bands (302) areconstant for each direction, i.e. S_(L) in the X-axis direction andS_(W) in the Y-axis direction. Therefore, the guard bands are placed atspecified spacings from the active regions regardless of the shape ofthe active regions, i.e. whether rectangular or not.

In one embodiment, the spacing S_(L) in the X-direction can be specifiedas the same spacing in the X-direction used in a Spice modelcharacterization of the integrated circuit structure. Similarly, thespacing S_(W) in the Y-direction can be specified as the same as thespacing in the Y-direction used in a Spice model characterization of theintegrated circuit structure. The advantage of this embodiment is thatthe actual diffusion patterns will have the same X- and Y-diffusionspacings as Spice model characterization spacings of the integratedcircuit structure, thus helping to more accurately match simulation andfabrication of the integrated circuits, as well as significantlyreducing the gap between post-layout and pre-layout simulation results.

FIG. 4 through 6 are top views of intermediate stages according to oneaspect of a method of adding dummy diffusion patterns, where the guardbands are conforming to the shapes of the active layers, and blocklayers are used to keep out dummy diffusion layers at specified spacingsfrom active layers.

FIG. 4 shows the same layout as FIG. 3, except that block layers 402,404, 406, and 408 (dotted lines) are used to keep out dummy diffusionlayers at specified spacings from active layers. The block layers (402,404, 406, and 408) may be used as in FIG. 4 according to one embodiment,or the guard bands 302 may be placed without using block layers as inFIG. 3 according to another embodiment. The block layers (402, 404, 406,and 408) are added before forming the guard bands 302, and no dummydiffusion patterns are added inside the block layer.

FIG. 5 shows the step of removing convex corners of the guard bands andthe step of cutting the guard band longer than a specified length fromthe layout in FIG. 4. Guard band convex regions 304 show convex cornersof guard bands 302 originally shown in FIG. 4. The guard band convexregions 304 are removed to avoid diffusion rounding, which could affectthe spacing and avoid too long diffusion length, which could violatedesign rules. The cutting does not affect the spacing. Guard band cutoutregions 306 show cutout part of the guard bands 302 originally shown inFIG. 4, because of the longer length than a specified length for guardbands, where the length is determined by design rules. The design rulesrestrict the length of diffusion regardless of its location, but thediffusion should not be cut at locations that result in changing thespacing.

In accordance with another aspect of the embodiment, the guard bands 302conforming to the shapes of the active layers 402, 404, 406, and 408 canbe placed, where the guard bands surround the active layers, the guardbands are spaced from the active layers at a first spacing S_(L) in theX-axis direction and at a second spacing S_(W) in the Y-axis direction,and there is no guard band at locations where convex regions 304 arenecessary in order to have a continuous guard band. This eliminates thestep of removing the convex regions 304 shown in FIG. 5 because theconvex regions 304 are not placed in the first place. Likewise, theguard bands 302 can be placed so that the length of guard bands 302 areequal to or less than a specified length, so that there is no need tocut the guard bands at regions 306 as shown in FIG. 5, because theregions 306 are not placed in the first place.

FIG. 6 shows the step of adding dummy diffusion patterns from the layoutshown in FIG. 5. The general dummy diffusion patterns 14 are added inthe remaining space of the chip outside the guard band. The generaldummy diffusion patterns 14 have different granularities as shown inFIG. 6 according to one embodiment, and may have the same granularity inanother embodiment. Also, the general dummy diffusion patterns 14 can beadded so that the diffusion density is substantially uniform over thechip.

The advantageous features of the embodiments include well-controlled STIwidths or oxide definition (OD) spacing of devices, actual diffusionpatterns having the same X- and Y-diffusion spacings as a Spice modelcharacterization spacings if the same spacings are specified, andsignificantly reduced device performance variations due to STI stresseffect variations regardless of diffusion shapes.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations could be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A method of forming an integrated circuit structure on a chip, themethod comprising: extracting an active layer from a design of theintegrated circuit structure, wherein the active layer comprises anactive pattern having a diffusion region; forming, using a processor, atleast one guard band conforming to a shape of the active layer, whereinthe guard band is a dummy diffusion layer, the guard band surrounds theactive layer without a break, and the guard band is spaced from theactive layer at a first constant spacing in a X-axis direction and at asecond constant spacing in a Y-axis direction, which is perpendicular tothe X-axis direction; removing any part of the guard band that violatesdesign rules; removing convex corners of the guard band; and addingdummy diffusion patterns into a remaining space of the chip outside theguard band.
 2. The method of claim 1, further comprising adding a blocklayer before forming the guard band, wherein no dummy diffusion layersare added in the block layer.
 3. The method of claim 1, wherein thefirst constant spacing is the same as a third constant spacing in theX-direction used in a Spice model characterization of the integratedcircuit structure.
 4. The method of claim 1, wherein the second constantspacing is the same as a fourth constant spacing in the Y-direction usedin a Spice model characterization of the integrated circuit structure.5. The method of claim 1, further comprising cutting the guard band thatis longer than a specified length, wherein the length is determined bydesign rules.
 6. The method of claim 1, wherein the dummy diffusionpatterns have different granularities.
 7. The method of claim 1, whereinthe dummy diffusion patterns are added so that a diffusion density issubstantially uniform over the chip.
 8. A method of forming anintegrated circuit structure on a chip, the method comprising:extracting an active layer from a design of the integrated circuitstructure, wherein the active layer comprises an active pattern having adiffusion region; forming, using a processor, at least one guard bandconforming to a shape of the active layer, wherein the guard band is adummy diffusion layer, the guard band surrounds the active layer withouta break, and the guard band is spaced from the active layer at a firstconstant spacing in a X-axis direction and at a second constant spacingin a Y-axis direction, which is perpendicular to the X-axis direction,wherein the first constant spacing and the second constant spacing arethe same values used in a Spice model characterization; removing anypart of the guard band that violates design rules; removing convexcorners of the guard band; and adding dummy diffusion patterns into aremaining space of the chip outside the guard band.
 9. The method ofclaim 8, further comprising adding a block layer before forming theguard band, wherein no dummy diffusion patterns are added in the blocklayer.
 10. The method of claim 8, further comprising cutting the guardband that is longer than a specified length, wherein the length isdetermined by design rules.
 11. The method of claim 8, wherein the dummydiffusion patterns have different granularities.
 12. The method of claim8, wherein the dummy diffusion patterns are added so that a diffusiondensity is substantially uniform over the chip.
 13. A method of formingan integrated circuit structure on a chip, the method comprising:extracting an active layer from a design of the integrated circuitstructure, wherein the active layer comprises an active pattern having adiffusion region; forming, using a processor, at least one guard bandconforming to a shape of the active layer, wherein the guard band is adummy diffusion layer, the guard band surrounds the active layer, theguard band is spaced from the active layer at a first constant spacingin a X-axis direction and at a second constant spacing in a Y-axisdirection, which is perpendicular to the X-axis direction, and there isno guard band at locations where convex corners are necessary in orderto have a continuous guard band; and adding dummy diffusion patternsinto a remaining space of the chip outside the guard band.
 14. Themethod of claim 13, further comprising removing any part of the guardband that violates design rules.
 15. The method of claim 13, furthercomprising adding a block layer before forming the guard band, whereinno dummy diffusion patterns are added in the block layer.
 16. The methodof claim 13, wherein the first spacing is the same as a third constantspacing in the X-direction used in a Spice model characterization of theintegrated circuit structure.
 17. The method of claim 13, wherein thesecond spacing is the same as a fourth constant spacing in theY-direction used in a Spice model characterization of the integratedcircuit structure.
 18. The method of claim 13, wherein the guard band issplit if the guard band is longer than a specified length, wherein thelength is determined by design rules.
 19. The method of claim 13,wherein the dummy diffusion patterns have different granularities. 20.The method of claim 13, wherein the dummy diffusion patterns are addedso that a diffusion density is substantially uniform over the chip.